Coated and cured proppants

ABSTRACT

Solid proppants are coated with a phenol-urethane coating in one or more layers by a method comprising coating a proppant solid and then curing the coated proppant under conditions sufficient to substantially cure said proppant, wherein said coating comprises a substantially homogeneous mixture of (i) an isocyanate component having at least 2 isocyanate groups, (ii) an amine reactant, and optionally (iii) an amine that is a latent curing agent for said isocyanate.

FIELD OF INVENTION

The invention relates to a method for the production of coatedproppants, and also to the proppants obtained according to this method,to the uses thereof and to methods which use the proppants.

BACKGROUND OF THE INVENTION

Well fracturing is an often used technique to increase the efficiencyand productivity of oil and gas wells. Overly simplified, the processinvolves the introduction of a fracturing fluid into the well and theuse of fluid pressure to fracture and crack the well strata. The cracksallow the oil and gas to flow more freely from the strata and therebyincrease production rates in an efficient manner.

There are many detailed techniques involved in well fracturing, but oneof the most important is the use of a solid “proppant” to keep thestrata cracks open as oil, gas, water and other fluids found in wellflow through those cracks. The proppant is carried into the well withthe fracturing fluid which itself may contain a variety of viscosityenhancers, gelation agents, surfactants, etc. These additives alsoenhance the ability of the fracturing fluid to carry proppant to thedesired strata depth and location. The fracturing fluid for a particularwell may or may not use the same formulation for each depth in thestrata.

Proppants can be made of virtually any generally solid particle that hasa sufficiently high crush strength to prop open cracks in a rock strataat great depth and temperatures of about 125° C. and higher. Sand andceramic proppants have proved to be especially suitable for commercialuse.

A proppant that is flushed from the well is said to have a high “flowback.” Flow back is undesirable. In addition to closure of the cracks,the flushed proppants are abrasive and can damage or clog valves andpipelines in downstream processing facilities.

Synthetic resin coatings can be used to impart a degree of adhesion tothe proppant so that flow back is substantially reduced or eliminated.Such resins can include phenol resin, epoxy resin, polyurethane-phenolresin, furane resin, etc. See published U.S. Patent Application Nos.2002/0048676, 2003/0131998, 2003/0224165, 2005/0019574, 2007/0161515 and2008/0230223 as well as U.S. Pat. Nos. 4,920,192; 5,048,608; 5,199,491;6,406,789; 6,632,527; 7,624,802; and published international applicationWO 2010/049467, the disclosures of which are herein incorporated byreference.

With some coatings, the synthetic coating is not completely cured whenthe proppant is introduced into the well. The coated, partially-curedproppants are pourable, but the coating resin is still slightlythermoplastic. The final cure is intended to occur in situ in the stratafracture at the elevated pressures and temperatures found “down hole.”

Unfortunately, partial curing of coatings on sand-sized proppants isextremely difficult to control in a reproducible manner. The stresses onpackage stacking, the temperatures experienced in warehouse storageduring storage and temperature stress upon introduction of the proppantsinto the strata can all raise the temperature sufficiently to causeuncontrolled post-curing at undesirable times. As one might imagine,such instability and handling difficulties have effectively limited theuse of post-cure coatings in proppants for oil and gas wells.

Proppants based on polyurethane chemistries have a number of potentialadvantages over phenol resin systems. Most notably, the reaction ratesused to make polyurethane coatings are generally faster than phenolresins, cure at lower temperatures and do not have gaseous emissionsthat require specialized recovery equipment. The coating step withpolyurethanes can be carried out at temperatures of about 10° C. toabout 50° C. Polyurethane coatings can also be performed without the useof solvents, whereas many of the known methods, as a rule, requireorganic solvents for the resinous coating. The components inpolyurethane systems are also generally easier to use and pose lowerenvironmental issues. These factors could reduce the cost to make coatedproppants and could also permit the coating process to be moved to thesite of the well head.

Polyurethanes have not, however, achieved widespread adoption due torelatively high flow back ratios at the down hole conditions and lowcoating levels needed to permit the proppant to enter the very smallfractures in a stratum. The coated proppant simply flows back up out ofthe well and does not become lodged therein in sufficient quantities tomaintain conductivity.

SUMMARY OF THE INVENTION

It would be desirable to have a polyurethane-based proppant coating thatwould enhance proppant retention within a fractured well field stratumwhile retaining good conductivity of the fractured well field.

It would also be desirable to have a polyurethane coated proppant thatretained its coating under the conditions prevailing within an activelyproducing well field stratum.

These and other objectives of the invention that will become apparentfrom the description herein can be accomplished by a polyurethanecoating and coating process that comprises the step of:

coating a proppant solid with a substantially homogeneous mixture of (i)an isocyanate component having at least 2 isocyanate groups, (ii) anamine reactant, and optionally (iii) an amine that is a latent curingagent for said isocyanate under conditions sufficient to substantiallycure said proppant coating.

A coated proppant according to the invention comprises a solid proppantcore particle that is substantially covered with a coating made from theat least substantially cured, reaction product of an isocyanatecomponent, an amine reactant and, optionally, an amine-based latentcuring agent and, optionally, a polyol.

The use of one or more amine-functional materials in the formulation ofthe coating permit the coating or each coating layer to become at leastsubstantially cured and thereby develop adequate resistance todissolution that the coating will remain adhered to the proppant solidat the rigorous conditions found at well depths. The additional heat andcuring time provided by the coating process of the present inventioncauses the polyurethane coating components to crosslink fully andthereby reduce what water solubility might remain in the coating fromunreacted components. The result is a substantially cured, polyurethanecoating that exhibits an acceptably low rate of flow back with a faster,less expensive coating process that requires less capital thancomparable phenol resin-based coatings.

DETAILED DESCRIPTION OF THE INVENTION

The coating formulation of the present invention includes asubstantially homogeneous mixture that comprises: (a) an isocyanatereactant, (b) a polyol reactant which may or may not have reactive aminefunctionality, (c) an amine reactant, and (d) optionally an amine-based,latent curing agent.

The coating process of the present invention applies one or more layersof cured polyurethane around a solid proppant core that is substantiallycured and crosslinked to resist dissolution under the rigorouscombination of high heat, agitation, abrasion and water found downholein a well. Preferably, the cured coating exhibits a sufficientresistance to a 10 day autoclave test or 10 day conductivity test sothat the coating resists loss by dissolution in hot water (“LOI loss”)of less than 25 wt %, more preferably less than 15 wt %, and even morepreferably a loss of less than 5 wt %. The substantially cured coatingof the invention thus resists dissolution in the fractured stratum whilealso exhibiting sufficient resistance to flow back and sufficiently highcrush strength to maintain conductivity of the fractures.

A preferred testing method for is described in ISO 13503-5:2006(E)“Procedures for measuring the long term conductivity of proppants”, thedisclosure of which is herein incorporated by reference. ISO13503-5:2006 provides standard testing procedures for evaluatingproppants used in hydraulic fracturing and gravel packing operations.ISO 13503-5:2006 provides a consistent methodology for testing performedon hydraulic fracturing and/or gravel packing proppants. The “proppants”mentioned henceforth in this part of ISO 13503-5:2006 refer to sand,ceramic media, resin-coated proppants, gravel packing media, and othermaterials used for hydraulic fracturing and gravel-packing operations.ISO 13503-5:2006 is not applicable for use in obtaining absolute valuesof proppant pack conductivities under downhole reservoir conditions, butit does serve as a consistent method by which such downhole conditionscan be simulated and compared in a laboratory setting.

The Isocyanate Component

The isocyanate component comprises an isocyanate with at least 2reactive isocyanate groups. Other isocyanate-containing compounds may beused, if desired. Examples of suitable isocyanate with at least 2isocyanate groups an aliphatic or an aromatic isocyanate with at least 2isocyanate groups (e.g. a diisocyanate, diisocyanate ortetraisocyanate), or an oligomer or a polymer thereof can preferably beused. These isocyanates with at least 2 isocyanate groups can also becarbocyclic or heterocyclic and/or contain one or more heterocyclicgroups.

The isocyanate with at least 2 isocyanate groups is preferably acompound of the formula (III) or a compound of the formula (IV):

In the formulas (III) and (IV), A is each, independently, an aryl,heteroaryl, cycloalkyl or heterocycloalkyl. Preferably, A is each,independently, an aryl or cycloalkyl. More preferably A is each,independently, an aryl which is preferably phenyl, naphthyl oranthracenyl, and most preferably phenyl. Still more preferably A is aphenyl.

The above mentioned heteroaryl is preferably a heteroaryl with 5 or 6ring atoms, of which 1, 2 or 3 ring atoms are each, independently, anoxygen, sulfur or nitrogen atom and the other ring atoms are carbonatoms. More preferably the heteroaryl is selected among pyridinyl,thienyl, furyl, pyrrolyl, imidazolyl, pyrazolyl, pyrazinyl, pyrimidinyl,pyridazinyl, oxazolyl, isoxazolyl or furazanyl.

The above mentioned cycloalkyl is preferably a C₃₋₁₀-cycloalkyl, morepreferably a C₅₋₇-cycloalkyl.

The above mentioned heterocycloalkyl is preferably a heterocycloalkylwith 3 to 10 ring atoms (more preferably with 5 to 7 ring atoms), ofwhich one or more (e.g. 1, 2 or 3) ring atoms are each, independently,an oxygen, sulfur or nitrogen atom and the other ring atoms are carbonatoms. More preferably the heterocycloalkyl is selected from amongtetrahydrofuranyl, piperidinyl, piperazinyl, aziridinyl, acetidinyl,pyrrolidinyl, imidazolidinyl, morpholinyl, pyrazolidinyl,tetrahydrothienyl, octahydroquinolinyl, octahydroisoquinolinyl,oxazolidinyl or isoxazolidinyl. Still more preferably, theheterocycloalkyl is selected from among tetrahydrofuranyl, piperidinyl,piperazinyl, pyrrolidinyl, imidazolidinyl, morpholinyl, pyrazolidinyl,tetrahydrothienyl, oxazolidinyl or isoxazolidinyl.

In the formulas (III) and (IV), each R¹ is, independently, a covalentbond or C₁₋₄-alkylene (e.g. methylene, ethylene, propylene or butylene).Preferably each R² is a covalent bond.

In the formulas (III) and (IV), each R² is each, independently, ahalogen (e.g. F, Cl, Br or I), a C₁₋₄ alkyl (e.g. methyl, ethyl, propylor butyl) or C₁₋₄ alkyoxy (e.g. methoxy, ethoxy, propoxy or butoxy).Preferably, each R² is, independently, a C₁₋₄-alkyl. More preferablyeach R² is methyl.

In the formula (N), R³ is a covalent bond, a C₁₋₄-alkylene (e.g.methylene, ethylene, propylene or butylene) or a group—(CH₂)_(R31)—O—(CH₂)_(R32-), wherein R31 and R32 are each,independently, 0, 1, 2 or 3. Preferably, R³ is a —CH₂- group or an —O-group.

In the formula (III), p is equal to 2, 3 or 4, preferably 2 or 3, morepreferably 2.

In the formulas (III) and (N), each q is, independently, an integer from0 to 3, preferably 0, 1 or 2. When q is equal to 0, the correspondinggroup A has no substitutent R², but has hydrogen atoms instead of R².

In the formula (N), each r and s are, independently, 0, 1, 2, 3 or 4,wherein the sum of r and s is equal to 2, 3 or 4. Preferably, each r ands are, independently, 0, 1 or 2, wherein the sum of r and s is equal to2. More preferably, r is equal to 1 and s is equal to 1.

Examples of the isocyanate with at least 2 isocyanate groups are:toluol-2,4-diisocyanate; toluol-2,6-diisocyanate;1,5-naphthalindiisocyanate; cumol-2,4-diisocyanate;4-methoxy-1,3-phenyldiisocyanate; 4-chloro-1,3-phenyldiisocyanate;diphenylmethane-4,4-diisocyanate; diphenylmethane-2,4-diisocyanate;diphenylmethane-2,2-diisocyanate; 4-bromo-1,3-phenyldiisocyanate;4-ethoxy-1,3-phenyl-diisocyanate; 2,4′-diisocyanate diphenylether;5,6-dimethyl-1,3-phenyl-diisocyanate;2,4-dimethyl-1,3-phenyldiisocyanate; 4,4-diisocyanato-diphenylether;4,6-dimethyl-1,3-phenyldiisocyanate; 9,10-anthracene-diisocyanate;2,4,6-toluol triisocyanate; 2,4,4′-triisocyanatodiphenylether;1,4-tetramethylene diisocyanate; 1,6-hexamethylene diisocyanate;1,10-decamethylene-diisocyanate; 1,3-cyclohexylene diisocyanate;4,4′-methylene-bis-(cyclohexylisocyanate); xylol diisocyanate;1-isocyanato-3-methyl-isocyanate-3,5,5-trimethylcyclohexane (isophoronediisocyanate); 1-3-bis(isocyanato-1-methylethyl) benzol (m-TMXDI);1,4-bis(isocyanato-1-methylethyl) benzol (p-TMXDI); oligomers orpolymers of the above mentioned isocyanate compounds; or mixtures of twoor more of the above mentioned isocyanate compounds or oligomers orpolymers thereof.

Particularly preferred isocyanates with at least 2 isocyanate groups aretoluol diisocyanate, diphenylmethane diisocyanate, an oligomer based ontoluol diisocyanate or an oligomer based on diphenylmethanediisocyanate.

The Polyol Component

A polyol component can be added to the coating formulation. The polyolcomponent may or may not have reactive amine functionality. Anespecially preferred polyurethane coating is a phenolic polyurethanemade with a phenolic polyol according to a patent application that wasfiled with the German Patent Office under no. DE 10 2010 051 817.4 onNov. 19, 2010 and entitled “Proppant Coating Technology”, the disclosureof which is herein incorporated by reference and summarized below in thecontext of the process of the present invention.

Another preferred polyol component for the present process comprises aphenol resin that comprises a condensation product of a phenol and analdehyde, such as formaldehyde. The phenol resin is preferably a resoleor novolak phenol resin and more preferably a benzyl ether resin.

The resole-type phenol resin can be obtained, for example, bycondensation of phenol or of one or more compounds of the followingformula (I), with aldehydes, preferably formaldehyde, under basicconditions.

In the formula (I):

“R” is in each case, independently, a hydrogen atom, a halogen atom,C₁₋₁₆-alkyl (preferably C₁₋₁₂-alkyl, more preferably C₁₋₆-alkyl, andstill more preferably methyl, ethyl, propyl or butyl) or —OH;

“p” is an integer from 0 to 4, preferably 0, 1, 2 or 3, and morepreferably 1 or 2.

Those in the art will understand that when p is 0, the compound offormula (I) is phenol.

Novolak-type phenol resin for the present invention comprises thecondensation product of phenol or of one or more compounds of theformula (I) defined above, with aldehydes, preferably formaldehyde,under acidic conditions.

In another preferred embodiment, the phenol resin is a benzyl etherresin of the general formula (II):

In the formula (II):

A, B and D each are, independently, a hydrogen atom, a halogen atom, aC₁₋₁₆-hydrocarbon residue, -(C₁₋₁₆-alkylene)-OH, —OH, an—O-(C₁₋₁₆-hydrocarbon residue), phenyl, -(C₁₋₆-alkylene)-phenyl, or-(C₁₋₆-alkylene)-phenylene-OH;

The halogen atom is F, Cl, Br or I;

The C₁₋₁₆-hydrocarbon-residue is preferably C₁₋₁₆-alkyl, C₂₋₁₆-alkenylor C₂₋₁₆-alkinyl, more preferably C₁₋₁₂-alkyl, C₂₋₁₂-alkenyl orC₂₋₁₂-alkinyl, still more preferably C₁₋₆-alkyl, C₂₋₆-alkenyl orC₂₋₆-alkinyl, and still more preferably C₁₋₄-alkyl, C₂₋₄-alkenyl orC₂₋₄-alkinyl, and still more preferably C₁₋₁₂-alkyl, and still morepreferably C₁₋₆-alkyl, and still more preferably methyl, ethyl, propylor butyl, and most preferably methyl;

The residue -(C₁₋₁₆-alkylene)-OH is preferably -(C₁₋₁₂-alkylene)-OH,more preferably -(C₁₋₆-alkylene)-OH, and still more preferably-(C₁₋₄-alkylene)-OH, and most preferably a methylol group (—CH₂—OH);

The —O-(C₁₋₁₆-hydrocarbon)-residue is preferably C₁₋₁₆-alkoxy, morepreferably C₁₋₁₂-alkoxy, and still more preferably C₁₋₆-alkoxy, andstill more preferably C₁₋₄-alkoxy, and still more preferably —O—CH₃,—O—CH₂CH₃, —O—(CH₂)₂CH₃ or —O—(CH₂)₃CH₃;

The residue -(C₁₋₆-alkylene)-phenyl is preferably-(C₁₋₄-alkylene)-phenyl, and more preferably —CH₂-phenyl;

The residue -(C₁₋₆-alkylene)-phenylene-OH is preferably-(C₁₋₄-alkylene)-phenylene-OH, and more preferably —CH₂-phenylene-OH;

R is a hydrogen atom of a C₁₋₆-hydrocarbon residue (e.g. linear orbranched

R is particularly preferred as a hydrogen atom. This is the case, forexample, when formaldehyde is used as aldehyde component in acondensation reaction with phenols in order to produce the benzyl etherresin of the formula (II);

m¹ and m² are each, independently, 0 or 1.

n is an integer from 0 to 100, preferably an integer from 1 to 50, morepreferably from 2 to 10, and still more preferably from 2 to 5; and

wherein the sum of n, m′ and m² is at least 2.

In a still further embodiment, the polyol component is a phenol resinwith monomer units based on cardol and/or cardanol. Cardol and cardanolare produced from cashew nut oil which is obtained from the seeds of thecashew nut tree. Cashew nut oil consists of about 90% anacardic acid andabout 10% cardol. By heat treatment in an acid environment, a mixture ofcardol and cardanol is obtained by decarboxylation of the anacardicacid. Cardol and cardanol have the structures shown below:

As shown in the illustration above, the hydrocarbon residue(—C₁₅H_(31-n)) in cardol and/or in cardanol can have one (n=2), two(n=4) or three (n=6) double bonds. Cardol specifically refers tocompound CAS-No. 57486-25-6 and cardanol specifically to compoundCAS-No. 37330-39-5.

Cardol and cardanol can each be used alone or at any particular mixingratio in the phenol resin. Decarboxylated cashew nut oil can also beused.

Cardol and/or cardanol can be condensed into the above described phenolresins, for example, into the resole- or novolak-type phenol resins. Forthis purpose, cardol and/or cardanol can be condensed e.g. with phenolor with one or more of the above defined compounds of the formula (I),and also with aldehydes, preferably formaldehyde.

The amount of cardol and/or cardanol which is condensed in the phenolresin is not particularly restricted and preferably is from about 1 wt %to about 99 wt %, more preferably about 5 wt % to about 60 wt %, andstill more preferably about 10 wt % to about 30 wt %, relative to 100 wt% of the amount of phenolic starting products used in the phenol resin.

In another embodiment, the polyol component is a phenol resin obtainedby condensation of cardol and/or cardanol with aldehydes, preferablyformaldehyde.

A phenol resin which contains monomer units based on cardol and/orcardanol as described above, or which can be obtained by condensation ofcardol and/or cardanol with aldehydes, has a particularly low viscosityand can thus preferably be employed with a low addition or withoutaddition of reactive thinners. Moreover, this kind of long-chain,substituted phenol resin is comparatively hydrophobic, which results ina favorable shelf life of the coated proppants obtained by the methodaccording to the present invention. In addition, a phenol resin of thiskind is also advantageous because cardol and cardanol are renewable rawmaterials.

Apart from the phenol resin, the polyol component can still containother compounds containing hydroxyl groups. The other compoundscontaining hydroxyl groups can be selected from the compounds containinghydroxyl groups that are known to be useful for making polyurethanes,e.g., hydroxy-functional polyethers, hydroxy-functional polyesters,alcohols or glycols. One preferred compound containing hydroxyl groupsis, for instance, castor oil. Compounds containing hydroxyl groups suchas alcohols or glycols, in particular cardol and/or cardanol, can beused as reactive thinners.

The amount of the other compounds containing hydroxyl groups depends onthe desired properties of the proppant coating and can suitably beselected by the person skilled in the art. Typical amounts of compoundscontaining hydroxyl groups are in the range of between about 10 wt % andabout 80 wt %, preferably from about 20 wt % to about 70 wt %, relativeto 100 wt % of the polyol component.

The process of the present invention is particularly useful when theproppants are coated with a condensation reaction product that has beenmade with an excess of isocyanate component with respect to the polyolcomponent. In step (a) therefore, 100 parts by weight of the polyolcomponent is used with about 105 wt % and about 300 wt %, preferablyabout 110 wt % to about 230wt %, more preferably about 120 wt % to about220 wt %, and still more preferably about 130 wt % to about 200 wt %, ofthe isocyanate base value.

The isocyanate base value defines the amount of the isocyanate componentwhich is equivalent to 100 parts by weight of the polyol component. TheNCO-content (%) of the isocyanate component is defined herein accordingto DIN ISO 53185. To determine the OH-content (%) of the polyolcomponent, first the so-called OH-number is determined in mg KOH/gaccording to DIN ISO 53240 and this value is divided by 33, in order todetermine the OH-content.

Thus, in step (a) an excess of NCO-groups in the isocyanate component ofbetween about 5 and about 200%, preferably about 10 to about 130%, morepreferably about 20% to about 120%, and still more preferably about 30%to about 100%, relative to the OH-groups in the polyol component is used(corresponding to the above mentioned amount of isocyanate component ofabout 105% to about 300%, preferably about 110% to about 230%, morepreferably about 120% to about 220%, still even more preferably about130% to about 200% of the isocyanate base value).

Moreover, in step (a) one or more additives can be mixed with theproppant, the polyol component and the isocyanate component. Theseadditives are not particularly restricted and can be selected from theadditives known in the specific field of coated proppants. Provided thatone of these additives has hydroxyl groups, it should be considered as adifferent hydroxyl-group-containing compound, as described above inconnection with the polyol component. If one of the additives hasisocyanate groups, it should be considered as a differentisocyanate-group-containing compound. Additives with hydroxyl groups andisocyanate groups can be simultaneously considered as differenthydroxyl-group-containing compounds and as differentisocyanate-group-containing compounds.

The Amine Component

The coating formulation of the present invention also includes areactive amine component, preferably an amine-terminated compound. Thiscomponent enhances crosslink density within the coating and, dependingon component selection, can provide additional characteristics ofbenefit to the cured coating. Particularly preferred reactive aminecomponents for use in the present invention include amine-terminatedcompounds such as diamines, triamines, amine-terminated glycols such asthe amine-terminated polyalkylene glycols sold commercially under thetrade name JEFFAMINE from Huntsman Performance Products in TheWoodlands, Tex.

Suitable diamines include primary, secondary and higher polyamines andamine-terminated compounds. Suitable compounds include, but are notlimited to, ethylene diamine; propylenediamine; butanediamine;hexamethylenediamine; 1,2-diaminopropane; 1,4-diaminobutane;1,3-diaminopentane; 1,6-diaminohexane; 2,5-diamino-2,5-dimethlhexane;2,2,4- and/or 2,4,4-trimethyl-1,6-diaminohexane; 1,11-diaminoundecane;1,12-diaminododecane; 1,3- and/or 1,4-cyclohexane diamine;1-amino-3,3,5-trimethyl-5-aminomethyl-cyclohexane; 2,4- and/or2,6-hexahydrotoluylene diamine; 2,4′ and/or 4,4′-diaminodicyclohexylmethane and 3,3′-dialkyl-4,4′-diamino-dicyclohexyl methanes such as3,3′-dimethyl-4,4-diamino-dicyclohexyl methane and3,3′-diethyl-4,4′-diaminodicyclohexyl methane; aromatic polyamines suchas 2,4- and/or 2,6-diaminotoluene and 2,6-diaminotoluene and 2,4′ and/or4,4′-diaminodiphenyl methane; and polyoxyalkylene polyamines (alsoreferred to herein as amine terminated polyethers).

Mixtures of polyamines may also be employed in preparing asparticesters, which is a secondary amine derived from a primary polyamine anda dialkyl maleic or fumaric acid ester, for use in the invention.Representative examples of useful maleic acid esters include dimethylmaleate, diethyl maleate, dibutyl maleate, dioctyl maleate, mixturesthereof and homologs thereof.

Suitable triamines and higher multifunctional polyamines for use in thepresent coating include diethylene triamine, triethylenetetramine, andhigher homologs of this series.

JEFFAMINE diamines include the D, ED, and EDR series products. The Dsignifies a diamine, ED signifies a diamine with a predominatelypolyethylene glycol (PEG) backbone, and EDR designates a highlyreactive, PEG based diamine.

JEFFAMINE D series products are amine terminated polypropylene glycolswith the following representative structure:

JEFFAMINE ® x MW* D-230 ~2.5 230 D-400 ~6.1 430 D-2000 ~33 2,000 D-4000(XTJ-510) ~68 4,000

JEFFAMINE EDR-148 (XTJ-504) and JEFFAMINE EDR-176 (XTJ-590) amines aremuch more reactive than the other JEFFAMINE diamines and triamines. Theyare represented by the following structure:

JEFFAMINE ® y x + z MW* HK-511 2.0 ~1.2 220 ED-600 (XTJ-500) ~9.0 ~3.6600 ED-900 (XTJ-501) ~12.5 ~6.0 900 ED-2003 (XTJ-502) ~39 ~6.0 2,000

JEFFAMINE T series products are triamines prepared by reaction ofpropylene oxide (PO) with a triol intiator followed by amination of theterminal hydroxyl groups. They are exemplified by the followingstructure:

Moles PO JEFFAMINE ® R n (x + y + z) MW* T-403 C₂H₅ 1 5-6 440 T-3000 (XTJ-509) H 0 50 3000 T-5000 H 0 85 5000

The SD Series and ST Series products consist of secondary amine versionsof the JEFFAMINE core products. The SD signifies a secondary diamine andST signifies a secondary trimine. The amine end-groups are reacted witha ketone (e.g. acetone) and reduced to create hindered secondary amineend groups represented by the following terminal structure:

One reactive hydrogen on each end group provides for more selectivereactivity and makes these secondary di- and triamines useful forintermediate synthesis and intrinsically slower reactivity compared withthe primary JEFFAMINE amines.

JEFFAMINE ® Base Product MW* SD-231 (XTJ-584) D-230 315 SD-401 (XTJ-585)D-400 515 SD-2001 (XTJ-576) D-2000 2050 ST-404 (XTJ-586) T-403 565

See also U.S. Pat. Nos. 6,093,496; 6,306,964; 5,721,315; 7,012,043; andPublication U.S. Patent Application No. 2007/0208156 the disclosure ofwhich are hereby incorporated by reference.

Optional Amine-Based Latent Curing Agents

Amine-based latent curing agents are optionally added to the coatingformulation in the isocyanate component, the polyol component, theamine-reactive polyol component or added simultaneously as any of thesecomponents or pre-coated on the proppant. Suitable amine-based latentcuring agents for use with the present invention includetriethylenediamine; bis(2-dimethylaminoethyl)ether;tetramethylethylenediamine; pentamethyldiethylenetriamine; and othertertiary amine products of alkyleneamines. Additionally, other catalyststhat promote the reaction of isocyanates with hydroxyls and amines thatare known by the industry can be used in the present invention.

Additives

The proppant coating compositions of the invention may also includevarious additives. For example, the coatings of the invention may alsoinclude pigments, tints, dyes, and fillers in an amount to providevisible coloration in the coatings. Other materials conventionallyincluded in coating compositions may also be added to the compositionsof the invention. These additional materials include, but are notlimited to, reaction enhancers or catalysts, crosslinking agents,optical brighteners, propylene carbonates, coloring agents, fluorescentagents, whitening agents, UV absorbers, hindered amine lightstabilizers, defoaming agents, processing aids, mica, talc, nano-fillersand other conventional additives. All of these materials are well knownin the art and are added for their usual purpose in typical amounts. Forexample, the additives are preferably present in an amount of about 15weight percent or less. In one embodiment, the additive is present in anamount of about 5 percent or less by weight of the coating composition.

Other additives can include, for example, solvents, softeners,surface-active agents, molecular sieves for removing the reaction water,thinners and/or adhesion agents can be used. Silanes are a particularlypreferred type of adhesion agent that improves the affinity of thecoating resin for the surface of the proppant. Silanes can be mixed inas additives in step (a), but can also be converted chemically withreactive constituents of the polyol component or of the isocyanatecomponent. Functional silanes such as amino-silanes, epoxy-, aryl- orvinyl silanes are commercially available and, as described above, can beused as additives or can be converted with the reactive constituents ofthe polyol component or of the isocyanate component. In particular,amino-silanes and epoxy-silanes can be easily converted with theisocyanate component.

Proppant Core Solids

The proppants can be virtually any small solid with an adequate crushstrength and lack of chemical reactivity. Suitable examples includesand, ceramic particles (for instance, aluminum oxide, silicon dioxide,titanium dioxide, zinc oxide, zirconium dioxide, cerium dioxide,manganese dioxide, iron oxide, calcium oxide or bauxite), or also othergranular materials. The proppants to be coated preferably have anaverage particle size within the range from about 50 μm and about 3000μm, and more preferably within the range from about 100 μm to about 2000μm.

Coating Method

The method for the production of coated proppants according to thepresent invention can be implemented without the use of solvents.Accordingly, the mixture obtained in step (a) in one embodiment of themethod is solvent-free, or is essentially solvent-free. The mixture isessentially solvent-free, if it contains less than 20 wt %, preferablyless than 10 wt %, more preferably less than 5 wt %, and still morepreferably less than 3 wt %, and most preferably less than 1 wt % ofsolvent, relative to the total mass of components of the mixture.

Preferably, the method is implemented without the use of organicsolvents. In this case, the mixture obtained in step (a) is free oforganic solvents, or is essentially free of organic solvents. Themixture is essentially free of organic solvents, if it contains lessthan 20 wt %, preferably less than 10 wt %, more preferably less than 5wt %, and still more preferably less than 3 wt %, and most preferablyless than 1 wt % of solvent, relative to the total mass of components ofthe mixture.

In step (a) the proppant is preferably heated to an elevated temperatureand then contacted with the coating components. Preferably, the proppantis heated to a temperature within the range of about 50° C. to about150° C. to accelerate crosslinking reactions in the applied coating

The mixer used for the coating process is not particularly restrictedand can be selected from among the mixers known in the specific field.For example, a pug mill mixer or an agitation mixer can be used. Forexample, a drum mixer, a plate-type mixer, a tubular mixer, a troughmixer or a conical mixer can be used. The easiest way is mixing in arotating drum. As continuous mixer, a worm gear can, for example, beused.

Mixing can be carried out on a continuous or discontinuous basis. Insuitable mixers it is possible, for example, to add adhesion agents,isocyanate, amine and optional ingredients continuously to the heatedproppants. For example, isocyanate components, amine reactant andoptional additives can be mixed with the proppant souds in a continuousmixer (such as a worm gear) in one or more steps to make one or morelayers of cured coating.

Preferably, the proppant, isocyanate component, amine reactant and theoptional additives are mixed homogeneously. Thus, the isocyanatecomponent and amine reactant are distributed uniformly on the surface ofthe proppants. The coating ingredients are preferably kept in motionthroughout the entire mixing process.

It is also possible to arrange several mixers in series, or to coat theproppants in several runs in one mixer.

The temperature of the coating process is not particularly restrictedoutside of practical concerns for safety and component integrity.Preferably, the coating step is performed at a temperature of betweenabout 10° C. and about 150° C., or more preferably at a temperature ofabout 10° C. to about 125° C.

The coating material may be applied in more than one layer. In thiscase, the coating process is repeated as necessary (e.g. 1-5 times, 2-4times or 2-3 times) to obtain the desired coating thickness. In thismanner, the thickness of the coating of the proppant can be adjusted andused as either a relatively narrow range of proppant size or blendedwith proppants of other sizes, such as those with more or less numbersof coating layers of polyurethane according to the present invention, soas to form a proppant blend have more than one range of sizedistribution. A typical range for coated proppant is typically withinthe range of about 20-70 mesh.

The amount of coating resin, that is, of the polyurethane resin appliedto a proppant, is preferably between about 0.5 and about 10 wt %, morepreferably between about 2 and about 5 wt %, resin relative to the massof the proppant as 100 wt %.

The coated proppants can additionally be treated with surface-activeagents or auxiliaries, such as talcum powder or stearate, to improvepourability.

If desired, the coated proppants can be baked or heated for a period oftime sufficient to substantially react at least substantially all of theavailable isocyanate, hydroxyl and reactive amine groups that mightremain in the coated proppant. Such a post-coating cure may occur evenif additional contact time with a catalyst is used after a first coatinglayer or between layers. Typically, the post-coating cure step isperformed like a baking step at a temperature within the range fromabout 100°-200° C. for a time of about 1-48 hours, preferably thetemperature is about 125°-175° C. for 19-36 hours.

Even more preferably, the coated proppant is cured for a time and underconditions sufficient to produce a coated proppant that exhibits a lossof coating of less than 25 wt %, preferably less than 15 wt %, and evenmore preferably less than 5 wt % when tested according to ISO13503-5:2006(E).

Using The Coated Proppants

Furthermore, the invention includes the use of the coated proppants inconjunction with a fracturing liquid for the production of petroleum ornatural gas. The fracturing liquid is not particularly restricted andcan be selected from among the frac liquids known in the specific field.Suitable fracturing liquids are described, for example, in W C Lyons, GJ Plisga, Standard Handbook Of Petroleum And Natural Gas Engineering,Gulf Professional Publishing (2005). The fracturing liquid can be, forexample, water gelled with polymers, an oil-in-water emulsion gelledwith polymers, or a water-in-oil emulsion gelled with polymers. In onepreferred embodiment, the fracturing liquid comprises the followingconstituents in the indicated proportions: 1000 l water, 20 kg potassiumchloride, 0.120 kg sodium acetate, 3.6 kg guar gum (water-solublepolymer), sodium hydroxide (as needed) to adjust a pH-value from 9 to11, 0.120 kg sodium thiosulfate, and 0.180 kg ammonium persulfate.

In addition, the invention relates to a method for the production ofpetroleum or natural gas which comprises the injection of the coatedproppant into the fractured stratum with the fracturing liquid, i.e.,the injection of a fracturing liquid which contains the coated proppant,into a petroleum- or natural gas-bearing rock layer, and/or itsintroduction into a fracture in the rock layer bearing petroleum ornatural gas. The method is not particularly restricted and can beimplemented in the manner known in the specific field.

With the method according to the present invention proppants can becoated at temperatures between about 10° C. and about 150° C. andpreferably in a solvent-free manner. The flow back effect can becontrolled and adjusted in a reproducible manner. The coating processrequires a comparatively little equipment and if necessary can also becarried out on a short-term basis in the vicinity of the bore.

EXAMPLES

Conductivity testing was performed at simulated downhole conditionsusing the method and procedures found in ISO 13503-5:2006. In suchtests, a closure stress is applied across a test unit for 50 hours toallow the proppant sample bed to reach a semi-steady state condition. Asthe fluid is forced through the proppant bed, the pack width,differential pressure, temperature and flow rates are measured at eachstress. Proppant pack permeability and conductivity are then calculated.

Multiple flow rates are used to verify the performance of thetransducers, and to determine Darcy flow regime at each stress; anaverage of the data at these flow rates is reported. The test fluid ispotassium chloride substitute solution filtered to 3 μm absolute. Theinitial conductivity, permeability and width is measured and compared tothe final conductivity, permeability and width after each stress period.Stress is applied and maintained using an Isco 260D. Stress is appliedat 100 psi/minute.

Width of the proppant pack is determined by assembling the conductivitycell with the Ohio sandstone wafers and shims without the sampleproppants. The distance between the width bars that are attached to eachend of the conductivity cells are measured at each of the four cornersand recorded. The cells are then assembled with the proppant samples.The measurements are made again at the beginning and ending of eachstress period. Width is determined by subtracting the average of thezero from the average of each of the stress width values. Conductivityis calculated using Darcy's equation.

-   -   Conductivity; kW_(f)=26.78 μQ/(ΔP)    -   Permeability; k=321.4 μQ/[(ΔP)W_(f)]        wherein:

-   k is the proppant pack permeability, expressed in Darcy's;

-   kW_(f) is the proppant pack conductivity, expressed in    millidarcy-feet

-   μ is the viscosity of the test liquid at test temperature, expressed    in centipoises;

-   Q is the flow rate, expressed in cubic centimeters per minute;

-   ΔP is the differential pressure, expressed in psi;

-   W_(f) is proppant pack width, expressed in inches.

Sieve analysis is performed using the procedure found in ISO 13503-2“Measurements of proppants used in hydraulic fracturing and gravel packoperations” Standard US mesh screens are used to separate the sample bysize. Not more than 0.1% should be greater than the first specifiedsieve and not more than 1% should be retained in the pan. There shouldbe at least 90% retained in the specified screens.

To determine the magnitude of “LOI” loss during the conductivity test,samples of the proppant pack are taken, dried in an oven and weighed.They are then subjected to a temperature of 960 C. for 2.5 hours. At theend of this period the samples are cooled and weighed again. Thedifference between the sample weight after drying but before beingsubjected to the furnace compared to the sample weight after the time inthe furnace, equates to the coating weight. Comparing this number to thesame test performed on a sample of the coated material before beingsubjected to the conductivity test, will equate to the coating weightlost due to the long term exposure to the conditions of the conductivitytests.

The procedure used in an autoclave test would be as follows:

The autoclave test utilizes what amounts to a pressure cooker to subjectthe coated sands to a hot wet environment that is above the boilingtemperature of water. Approximately 20 g of sample is placed in a jaralong with 150 ml of distilled water. The lids are placed on sample jarsbut not tightened. The samples are placed in the autoclave and thechamber is sealed. Heat is applied until the autoclave temperaturereaches 250-265° F. (121°-129° C.). The samples are maintained underthese conditions for the ten day period. At the end of the test periodthe autoclave is cooled down, opened and the sample jars removed. Eachsample is washed with distilled water and then placed in an oven to dry.The dried samples are then put through a standard test for determinationof LOI. This result is compared a the results of an LOI test that wasrun on the original sample. The difference in LOI before and after theautoclave test, quantifies the amount of LOI dissolved by the exposureto a hot water environment.

Example 1

One kg of sand is preheated to 210° F. (99° C.) and placed in a labmixer. The following components are added at the times indicated inTable 1 to proppant sand with mixing.

TABLE 1 Time (sec) Material(s) Added 0 Add 1 gm3-aminopropyltriemethoxysilane 15 Add 31.2 gms of a mixture of 60%methylene diphenylisocyanate (MDI) oligomers and 40% of a phenolicpolyol 45 Add 7.2 gms of a mixture of 96% JEFFAMINE D230 and 4%diazobicyclooctane (DABCO) 240 Remove the coated sand, now cooled to atemperature of 140° F. (60° C.)

The JEFFAMINE D230 is an amine-terminated, polypropylene glycol havingthe general structure:

-   -   wherein: x is about 2.5 and the average molecular weight is 230.

The final product exhibited excellent thermal properties and low or noloss of coating when immersed in water and subjected to a 250° F. (121°C.) autoclave test for 18 hours.

Example 2

One kg of sand is preheated to 210° F. (99° C.) and placed in a labmixer. The following components are added at the times indicated inTable 2 to the proppant sand with mixing.

TABLE 2 Time (sec) Material(s) Added 0 Add 1 gm3-aminopropyltriemethoxysilane 30 Add 20.0 gms of methylenediphenylisocyanate (MDI) oligomers 75 Add 12.0 gms of JEFFAMINE D230 240Remove the coated sand, now cooled to a temperature of 130° F. (54° C.)

The final, coated proppant product exhibited excellent thermalproperties and low/no loss of coating when immersed in water andsubjected to a 250° F. (121° C.) autoclave test for 18 hours.

Example 3

One kg of sand is preheated to 210° F. (99° C.) and placed in a labmixer. The following components are added at the times indicated inTable 3 to the proppant sand with mixing.

TABLE 3 Time (sec) Material(s) Added 0 Add 1 gm3-aminopropyltriemethoxysilane 30 Add 20.0 gms of methylenediphenylisocyanate (MDI) oligomers 75 Add 12.6 gms of a mixturecontaining 95 wt % JEFFAMINE D230 and 5 wt % of a latent crosslinkingagent comprising diazobicyclooctane (DABCO) 240 Remove the coated sand,now cooled to a temperature of 130° F. (54° C.)

The final product exhibited excellent thermal properties and low/no lossof coating when immersed in water and subjected to a 250° F. (121° C.)autoclave test for 18 hours.

Once those skilled in the art are taught the invention, many variationsand modifications are possible without departing from the inventiveconcepts disclosed herein. The invention, therefore, is not to berestricted except in the spirit of the appended claims.

1-24. (canceled)
 25. A coated proppant solid comprising a solid proppantcore particle substantially covered with an isocyanate-polyolcondensation reaction product, wherein the polyol is ahydroxy-functional polyether and the NCO-groups in the isocyanatecomponent is in an access from about 30% to less than 100%, relative tothe OH-groups in the polyol component.
 26. The coated proppant solid ofclaim 25, wherein the proppant solid exhibits a low rate of flow back.27. The coated proppant solid of claim 25, wherein the proppant solidhas sufficiently high crush resistance to maintain conductivity in asubterranean fracture.
 28. The coated proppant of claim 25, wherein thecoating further comprises an adhesion agent.
 29. The coated proppant ofclaim 25, wherein the coating further comprises a silane.
 30. The coatedproppant of claim 25, wherein the coating further comprises asurface-active agent.
 31. The coated proppant of claim 25, wherein thecoating further comprises a nanofiller.
 32. The coated proppant of claim25, wherein the solid proppant core particle is a ceramic core particle.33. The coated proppant of claim 25, wherein the solid proppant coreparticle is a sand particle.
 34. The coated proppant of claim 25,wherein the coating further comprises a pigment, dye, or tint.
 35. Thecoated proppant of claim 25, wherein the coating further comprises mica.36. A process for making the coated proppant solid of claim 25, theprocess comprising mixing a solid proppant core particle with anisocyanate and a polyol under conditions to coat the proppant solid witha isocyanate-polyol condensation reaction product, wherein the polyol isa hydroxy-functional polyether and the NCO-groups in the isocyanatecomponent is in an access from about 30% to less than 100%, relative tothe OH-groups in the polyol component.
 37. The process of claim 36,further comprising mixing the solid proppant core particle with anadhesion agent.
 38. The process of claim 36, further comprising mixingthe solid proppant core particle with a silane.
 39. The process of claim36, further comprising mixing the solid proppant core particle with asurface-active agent.
 40. The process of claim 36, further comprisingmixing the solid proppant core particle with a nanofiller.
 41. Theprocess of claim 36, wherein the solid proppant core particle is a sandparticle or a ceramic particle.
 42. The process of claim 36, furthercomprising mixing the solid proppant core particle with a pigment, dye,or tint.